As the rare usage of a fluid cylinder, there is the usage in which the fluid cylinder is provided between a member movably placed on, for example, a linear rail and a fixed member fixed outside the rail, and the member is moved on the rail by extension and contraction of the cylinder. In this usage, the cylinder extends and contracts, but its posture does not change.
On the other hand, in the usage frequently adopted, a pin hole in which the pin provided at another member is fitted is included in at least one of a rod front end portion and a tube rear end portion of the cylinder, and a tube (specifically, the cylinder itself) is relatively rotated around the pin during extension and contraction of the cylinder. In this usage, the posture of the cylinder changes during extension and contraction of the cylinder. In view of this usage, Japanese Utility Model Laid-open No. 1-148105 proposes a fluid cylinder with a swivel joint.
Summing it up, the art of the above-described Utility Model is “a double acting type and single rod type of fluid cylinder comprising a first opening in a pin outer circumferential surface, a second opening in a pin hole inner circumferential surface, which always opposes to the first opening even when a tube relatively rotates around the pin, a first flow passage leading from the first opening to a third opening provided in a pin end surface, a second flow passage leading from the second opening into the tube, and a swivel flow passage with which the first and second flow passages normally communicate via the first and second openings even when the tube relatively rotates around the pin,    wherein the pin comprising said first and third openings and the first flow passage is the pin fitted in the pin hole in a rear end portion of a tube,    said second opening is provided in an entire circumference of the pin hole inner circumferential surface, and    a flow passage, which leads to a chamber at a side opposite a rod in the tube, of said second flow passage is provided in a tube rear portion,    while a flow passage, which leads to a chamber at a rod side in the tube, of the second flow passage is provided by being connected with a pipe, which is extensively provided along a tube outer circumferential surface through holes provided in a front and rear portions of the tube”.
Incidentally, taking a hydraulic shovel being one of, for example, construction equipment frequently using fluid cylinders as an example, this equipment is frequently operated in cities and the like to say nothing of the operation on an uneven ground such as a mountain and forest, wasteland and the like. Accordingly, even though it is construction equipment, excellent appearance is demanded. Each hydraulic cylinder of a hydraulic shovel operates in a direction of specific normal load. Accordingly, if the art of the above-described Utility Model is adopted as it is, the following disadvantages occur.
(1) In the art of the above-described Utility Model, “the flow passage, which leads to the chamber at the rod side in the tube, of the second flow passage is provided by being connected with a pipe, which is extensively provided along the tube outer circumferential surface through holes provided in a front and rear portions of the tube”. Therefore, the pipe becomes an exposed object and projected object from a vehicle body to thereby impair the outer appearance. At the time of operations of dismantling structures in cities, various kinds of operations on an uneven ground and the like, it is feared that the projected object being the pipe collides against dismantled objects, branches, stones and rocks and the like to bring about non-operated vehicles with an unexpected trouble such as oil leakage.
(2) As for the manner in which the pin is fitted in the pin hole, they are strongly pressed to be in close contact with each other at a load side, and a gap occurs at a side opposite the load. However, in the art of the above-described Utility Model, “the second opening is provided in the entire circumference of the pin hole inner circumferential surface”. As a result, the following first and second disadvantages occur.
The first disadvantage is that as the operating time increases, the sliding surfaces of the pin and pin hole are worn, and when rattling occurs in the manner of fitting thereafter, a gap, at the side opposite the load, of the expanded gap causing the rattling makes it easy to directly leak oil to an outside from the second opening provided in the entire circumference of the pin hole inner circumferential surface.
The second disadvantage is caused by the fact that the hydraulic cylinder is composed of the flow passage leading to the chamber at the side opposite the rod in the tube and the flow passage leading to the chamber at the rod side. Specifically, both the second openings provided in the entire circumference of the pin hole inner circumferential surface cause both the flow passages to communicate with each other by the gap at the side opposite the load of the expanded gap causing the rattling.
Namely, “the sealing function from the outside and sealing function among the individual flow passages” of the swivel joint is impaired, and as a result, the disadvantage of reduction in extension and contraction ability of the cylinder occurs.